Cancer cells undergo dramatic changes in carbohydrate expression during the onset and progression of disease. As a result, there has been considerable interest in understanding why these changes occur, how they contribute to the disease, and exploiting these changes for clinical use. Carbohydrate binding antibodies and lectins play a critical role in this research for several reasons. First, antibodies and lectins are used extensively to detect and monitor changes in carbohydrate expression. In this capacity, they are used for basic research, as diagnostics, and as therapeutic agents. A comprehensive understanding of their specificity is critical for proper use and interpretation of results. Until recently, this information was not readily available. With the advent of carbohydrate microarray technology, or glycan arrays, we can now rapidly analyze binding of these proteins to a wide variety of carbohydrate antigens. In the past, we have screened a large set of commercially available lectin and antibody reagents used in the research community. We found that many of these reagents did not display the expected specificity. The information we provided regarding specificity will enable better use of these reagents and facilitate interpretation of results. In addition, the array can be used to develop new antibodies and lectins for monitoring new carbohydrate antigens. The array is being used to develop new monoclonal antibodies.